According to this article, recent rersearch notes parents will struggle with their mental health even when their children are adults, if the child is continuing to exhibit problematic behavior. While parents will report improved mental health functioning if they have an adult child who is successful, such gains are eliminated if even one of their adult children is having a problem, such as legal problems, substance abuse issues, etc. From the article:
"What this study finds is that the children may have their own lives and moved on, but their ups and downs are still deeply affecting their parents," psychology professor Karen Fingerman, PhD, said August 12 at the 118th Annual Convention of the American Psychological Association. Fingerman, of Purdue University, presented her findings at a symposium focusing on social relationships and well-being.
The researchers then looked at how children's successes and failures affected parents' well-being. Parents who had more than one highly successful child reported better well-being. However, having even one problematic child had a negative impact on their mental health, even if the other children were successful. Simply having at least one successful child was not associated with better well-being. The findings suggest parents react more strongly to their children's failures than their successes, according to Fingerman.
"Having two children suffering problems may be more demanding than having only one child who suffers problems," she said. "By the same token, having a successful child did not buffer the effects of problem-ridden children."
I think you tend to see this a lot, even in literature, movies, etc. (the movie Parenthood popped into mind while reading this). It seems like no matter how well some of the kids are doing, if one is struggling with something, that is the focus of the parents; not just in terms of problem-solving and assistance, but also emotional investment. Read the whole thing.